How to Pick the Right Fieldbus Protocol

There are four leading types of protocols commonly used in process plants: HART, PROFIBUSPA, FOUNDATION Fieldbus and WirelessHART. All four of these protocols are designed to connect field instruments to control and monitoring host systems, typically a distributed control system (DCS) or an asset management system (Figure 1).

The main differences among the four protocols are the degree of functionality and corresponding complexity. Wired HART instruments typically only deliver one variable and have limited diagnostics, and implementation is relatively simple. PROFIBUS PA, FOUNDATION Fieldbus and WirelessHART instruments supply a number of variables and diagnostics messages, which can add to the implementation challenge. WirelessHART instruments also supply many variables and diagnostics, and do so with a degree of difficulty that falls closer to wired HART than FOUNDATION Fieldbus and PROFIBUS.

Each of the four protocols has its advantages, design considerations and maintenance challenges.

Wired HART

Wired HART was one of the first digital communication protocols and is the most widely used. The HART signal is superimposed on the 4-20mA wiring already in place to connect the field instrument to a host system or configuration tool (Figure 2). This makes it easy to install as no new wiring, power supplies, or conditioners are required.

Where a standard 4-20mA instrument exists, an equivalent HART instrument is almost always available, so upgrading the instrument is simple. Depending on which host system is used, some modifications may be required to accept and interpret the HART signal.

HART has certain advantages:

Multiple variables—More than one variable can be transmitted from the instrument to the host system. Examples include process pressure and ambient temperature.

Remote information—Status and diagnostics information is available remotely.

Installation—Uses existing 4-20mA wiring.

Inferred Variables—While pressure, for example, is a direct measurement from a pressure transmitter, an inferred variable is a calculated measurement, such as flow or level from the same pressure transmitter. All HART instruments contain a microprocessor, so calculation of inferred variables can be added to the instrument. More specifically, a differential pressure instrument could measure pressure drop across a primary element, and then calculate or infer the volumetric flow rate. A differential pressure device installed on a tank would measure pressure in engineering units, such as in H20 or mbar, and calculate or infer tank level, tank volume, or a physical level unit such as inches or millimeters.

SIS approval—Approved for many safety-related applications.

Design considerations:

Secondary variable access—The instrument may make multiple measurements and infer multiple variables to produce multiple process variables, but the 4-20mA output can only represent one variable. HART 5 specifies a device can have up to four variables, and HART 7 specifies that a device can have up to eight variables. A device is therefore required to strip out the other variables in addition to the main variable and put them in a usable format. There are various ways to accomplish this. The host system may have the capability to host a HART interface card, but these cards can be expensive and slow to process the added variables. Another option is a HART splitter installed in series with the 4-20mA signal, with the signal passing through it. The HART splitter strips off the added variables from the HART signal and turns them into multiple 4-20mA signals. This approach is more expensive as it requires additional 4-20mA analog inputs at the host system, and wiring for each of these signals from the splitter to the host system. The third option is to use a wireless adapter to convert additional variables to a wireless signal. This option works especially well when there is an existing wireless infrastructure already in place.

Difficult to get diagnostics— The same challenges and options apply as getting secondary variables where actionable decisions are made. Diagnostic information is available, but one must typically be connected to the instrument, and it can be difficult to interpret this information.

Data transmission—Rates are very slow.

Host system support—Not all host systems support HART and some only support it partially, but it is the most widely supported protocol.

Non-deterministic—Data transmission rates can vary by the type of I/O card and amount of data traffic.

On-scale failures—An on-scale failure occurs when an instrument’s output is between 4 and 20mA but isn’t correct; for example, the output is at 8.2mA but it really should be 6mA. There are two common 4-20mA device failures that can cause on-scale failures. The first is a brown-out condition. All HART instruments have a published minimum voltage requirement, which is required to produce the maximum mA value, typically 20mA. If the voltage falls below the minimum, the instrument will have a correct output at the lower end of the 4-20mA scale, but as the mA output increases more voltage is dropped across the loads in the loop, leaving less for the instrument. When there is not enough voltage, the instrument cannot attain its full-scale 20mA output, and intermediate output values aren’t correct. The other on-scale failure occurs when a HART instrument has a quiescent current problem. Each HART instrument needs to be able to generate an output of 3.6mA to indicate a low failure alarm. This means the device can never draw more than 3.4mA to operate. A quiescent current problem could cause the HART instrument to draw excessive current to operate, for example 7mA, which means the device would have a minimum output of 7mA. All 4-20mA devices in SIS applications must have a proof test to check for these failures.

Grounding loops and issues—HART is a digital signal superimposed on the 4-20mA signal. If the shield of the 4-20mA cable is not grounded correctly, a ground loop is created that can cause electrical noise which is seen on the 4-20mA signal as jitter. This noise can affect the HART signal.

Maintenance challenges:

Keeping the HART communicator configuration tool current—It must be frequently upgraded to match the revision level of the instruments to which it is communicating.

Limited troubleshooting capability—HART diagnostic data is usually in the form of cryptic messages such as “User 4K EPROM Error,” which is confusing for a field technician using a configuration tool. Some host systems and configuration tools translate these messages to plain English, but many do not. The HART standard has also added several redundant device alerts, so one event can cause several alerts, possibly generating an alert flood.

The 4-20mA signal is an industry standard and is proven in use to the point of acceptance in safety applications requiring SIS approvals. It is simple to add HART functionality to existing 4-20mA installations, and this is the least expensive option overall for adding functionality such as additional variables, ease of calibration and maintenance—particularly for retrofit applications.

The HART caveats are the slower transmission rates and the difficulty of getting additional variables out of the instrument. The digital data is limited to situations where real-time and/or deterministic data is not required.

PROFIBUS PA

The PROFIBUS PA protocol is related to the more familiar and widely used PROFIBUS DP standard (Figure 3). PROFIBUS DP is typically used in machine automation and other discrete signal applications. PROFIBUS PA is used for measurement and process control applications and can be used in hazardous areas.

PROFIBUS PA has the same physical layer as FOUNDATION Fieldbus, but it uses master/slave communications and thus has non-deterministic data transfer. Digital protocols such as PROFIBUS PA confer certain advantages, but also add design considerations. Different types of instruments use this protocol, and here are its main advantages:

Multiple variables—More than one variable can be transmitted from the instrument to the host. For example, process pressure and ambient temperature.

Diagnostics—Status and diagnostics information is available remotely.

Speed—Fast access to device measurements and diagnostics.

Multiple devices—More than one instrument, valve or other device can be connected on the same pair of wires or segment.

Intrinsic safety applications—Few barriers needed because power levels are low.

Design considerations:

Extensive upfront design requirements—Many decisions have to be made during upfront design including hazardous area approvals such as fieldbus intrinsically safe concept (FISCO) or fieldbus non-incendive concept (FNICO). Other considerations include trunk power, the number of devices allowed on any one segment, bricks or no bricks, wiring architecture (trunk, spurs), short circuit protection, coupler or linking devices, etc.

Limited customization—No custom blocks/functionality are allowed.

Wiring design—Multiple guidelines need to be taken into account, such as the size of the wire, each instrument’s current draw, cable run lengths and other factors to determine whether each segment complies with requirements.

Conditioned power supply—All digital protocols require conditioned or very clean power to operate properly. Standard power supplies will not work as direct connection to these power supplies can disable the digital signal.

Short circuit protection—Digital bus systems such as PROFIBUS PA can host multiple instruments on a segment or one pair of wires. Short circuit protection is advisable to make sure if one instrument fails due to an electrical short, it doesn’t interrupt power to the other instrument on the segment.

Configuration files—There are two types of required files. The GSD files describe the functionality of the instrument, and the DD file defines communication parameters. They must be obtained for each instrument.

Strict grounding rules must be followed—Electrical noise generated by poor grounding can cause complete loss of the digital signal to and from the instrument.

Physical addressing—Suppliers usually ship all instruments with the same address. Each instrument on a segment requires a unique address, so the user must connect one device at a time and assign addresses during installation.

PA-to-DP link or segment coupler may be required—For installations with both PROFIBUS PA and PROFIBUS DP, couplers devices are required because PROFIBUS DP segments can run from 9.6 kbit/s to12 Mbit/s and PA segments run at 31.25 kbit/s data transmission rates. One of these two couplers must be installed where the PA bus splices into the DP bus. The segment coupler is a pass-through component and makes the devices transparent to the host system as data pass through it. The drawback to the segment coupler is that it usually requires the baud rate on the DP bus to be lowered. The PA-to-DP link is like a gateway or concentrator. It polls the devices independently of the system and caches the device data, and the host system polls the link for the cached data.

Termination—PROFIBUS PA segments require two terminators (resistor and capacitor) at each end of the segment.

Interoperability of profiles—The host system needs to support the profile of the instrument, currently Profile 3.

Non-deterministic data—Although faster than HART, data transmission is also not deterministic, especially when using a link or segment coupler.

Limited host system support—Among the four protocols discussed in this article, PROFIBUS PA has the fewest number of host systems supporting the protocol. And even in systems supporting it, the connection has to be made through a coupler or link device to a PROFIBUS DP card.

Maintenance challenges:

Training—Extensive training is required for installation and use.

New troubleshooting tools—These tools must be mastered.

Diagnostic bus tools—Basic electrical meters often aren’t sufficient to diagnose digital network problems, so a bus analyzer or an oscilloscope is frequently needed to analyze data and waveform shapes.

Limited configurators—There are just a few options in terms of available handheld configurators, and most configuration has to take place on a live segment.

Instrument revision change—If an instrument needs to be replaced and the new version has a different revision, the user must obtain and make sure to use the new device description (DD) or the instrument won’t work.

Difficult to pre-configure or troubleshoot on bench—There are very few PROFIBUS PA bench tools on the market, so most configuration is done by connecting the instrument to the control or monitoring system.

Block Modes—The technician needs to understand blocks and their modes, such as Target Mode, Actual Mode, Permitted Mode, Normal Mode, Out of Service, Local Override, Manual, Automatic and Remote Cascade.

FOUNDATION Fieldbus

FOUNDATION Fieldbus (FF) is the most widely deployed advanced fieldbus protocol, with millions of FF instruments installed worldwide. Its communications structure differs from PROFIBUS PA as it is peer-to-peer. This means instruments can communicate with each other, and can perform real-time control in the field as the data is deterministic. Its data structure differs from PROFIBUS as it allows custom blocks, and calculated or inferred variables.

FF was designed from the ground up as a high-speed, real-time instrument bus (Figure 4). Its strengths are:

Multiple variables—Multiple process, calculated or inferred variables can be transmitted from the instrument to other instruments or the host.

Custom blocks— Standard function blocks can be customized to perform local operations such as filtering; HI, HI-HI, LO, LO-LO alarming; and other calculations.

Deterministic data—All data is delivered in a predictable and consistent fashion.

Multiple devices on same wire—More than one instrument, valve or other device can be connected on the same segment.

Design considerations:

Block standardization—There are three different types of blocks in an FF instrument: resource, transducer and function. There is some industry-wide standardization for resource blocks and function blocks, but only some transducer blocks are defined by the foundation. It is up to the manufacturer to define those not defined by the foundation.

Scheduling blocks—Each device has one resource block, and one or more transducer blocks. These blocks are running all the time. The function blocks don’t run until they are scheduled to be used. So, for taking the output of an AI block and feeding it into an arithmetic block, for example, the user will not see an output of the arithmetic block until it is scheduled to be run in a macrocycle.

Link active scheduler (LAS) and backup LAS—The LAS is the traffic cop or director of the segment. There is only one pair of wires per segment and only one device can be communicating at any given time, so the LAS controls which block is communicating by running the schedule and all the support communications. The backup LAS keeps a copy of all the segment schedules, so if the LAS is lost, the backup LAS will automatically take over. Some manufacturers now ship FF instruments with input and output blocks already scheduled, so they work out of the box. A segment can have more than one backup, if desired.

Macrocycle— This is the control schedule, set up automatically by the control system depending on the blocks being used, the execution time of each block, and other factors.

Wiring design—As with PROFIBUS PA, users have to take into account parameters such as the size of the wire, each instrument’s current draw, and all the cable run lengths to determine whether each segment complies with requirements. Multiple software packages available to assist with these efforts.

Blocks in and out of service—An FF instrument can easily have 10 blocks, many have over 20. As with the blocks in a control system, each block has many modes such as Target, Permitted, Actual, Automatic, Manual, Out-of-Service, etc. Users must understand each instrument’s blocks and modes. As the instrument technicians may not be familiar with function blocks, they must learn the terminology to work on resource and transducer blocks. A device must be set in Out-of-Service mode to be calibrated, for example.

Configuration/device description files—As part of the instrument’s certification, each is assigned driver and configuration files. Users must make sure they have the correct device type and revision.

Instrument revision change—If an instrument needs to be replaced and the new version has a different revision, the user must obtain and make sure to use the new device description (DD), or the instrument won’t work.

Termination—As with PROFIBUS PA, the network is RS-485 and requires a termination resistor at each end of a segment.

Interoperability—Testing for certification is limited to functionality and doesn’t include formal interoperability testing. In the past, control systems were not tested and there were interoperability problems because they did not support certain device features. Even though the FieldComm GroupTM has implemented a more stringent testing for device and system interoperability, one must typically go to the vendor to get the device driver interoperable with their system.

Maintenance challenges:

Training—Extensive training is required for installation and use.

New troubleshooting tools—These tools must be mastered.

Limited configurator functionality—Handheld configurators are available for local interaction with the FF devices, but functionality can be limited to only certain blocks.

Calibration scheduling—With analog instruments, calibration is relatively easy due to the simplicity of the instruments. In the fieldbus world, the instruments are more powerful, so calibration is more difficult. For example, the device measurement block is the transducer block. A pressure can be applied to an FF pressure instrument, and the output of the transducer block can be examined. But, the transducer block output goes to an analog input block which publishes it to the rest of the devices. The AI block has functionality that can affect the published data such as linear/square root circuitry, damping circuitry, and a Lo, Lo-lo, Hi, Hi-Hi analog input block. The blocks must be scheduled, so the technician needs to use a tool with LAS capability.

Calibration tools—Different tools are available, and the right one must be selected. Tools that don’t support methods (wizards) require extensive technician training. The tech needs to know that calibration happens in the transducer block. He or she needs to find the block, know how to take it Out of Service, know how to find the units of calibration, and know parameter names such as PV_OUT to look at pressure being applied. Simple functions such as zeroing a pressure transmitter can be complicated without the right tools.

Responsibility/job description lines—With conventional analog instruments, responsibility lines used to follow the hardware: Control system personnel worked on controls, and instrument techs worked on instruments. But with FF, control functionality is provided with the instrument, providing much greater functionality, but also requiring a redefinition of responsibilities in many process plants.

WirelessHART

WirelessHART is a self-organizing and self-healing network of instruments communicating wirelessly. The power of this network is combined with the simplicity of the HART protocol. Although mostly touted for its low installation cost, this protocol also features easy access to all process variables, health status, diagnostic information, calculated/inferred variables, and network parameters. This data makes it well suited to reliability, maintenance, quality and other applications (Figure 5).

Other advantages include:

Wiring—None required for data transmission. Some wireless devices have external power options, typically devices requiring more power. The wires are for power only. The data is still transmitted. The other alternative for power is the use of energy harvesting devices to supply power. There is some wiring involved for power, data is still transmitted

Installation—Lowest cost and fastest installation.

Multiple variables—More than one variable can be transmitted from the instrument or valve to the host system. For example, valve position, deviation, actuator pressure, and ambient temperature could all be transmitted.

Diagnostics—Status and diagnostics information is readily available.

Control — Wireless has control capability. The measurement is required to go to the host which will drive the outputs. Although not as fast as FF, speed is sufficient for many real-time process control applications.

Easy access to data—Information is sent to the gateway which supports multiple wired protocols including Modbus TCP/IP, Modbus RTU, OPC, and Ethernet/IP.

HART familiarity—The user interface, maintenance and connectivity are very similar to wired HART.

Flexibility of installation—Can be easily installed in difficult to reach areas, and new instruments can be quickly added once the wireless mesh network infrastructure is in place.

Wireless repertoire continuously expanding—New wireless instruments/measurement types are being added continuously. Some of the newest are corrosion/erosion monitoring and H2S monitoring.

HART instruments are light, compact devices and are therefore easy to mount.

Secure encrypted communications.

Ability to pick data and diagnostics—Prevents operator data or diagnostic overload because each user can select the variables and diagnostics important to the process.

Some design considerations apply:

Getting the data out of the gateway—Must hardwire the gateway to the host system and make sure the host system supports the available gateway protocols.

Variable and diagnostic information selection—WirelessHART instruments can supply a very large number of variables and diagnostic messages. The user needs to select the information of interest from each instrument, and map that data to the gateway registers for output to the host system.

Wireless network best practices—Available from manufacturers, but are typically not established in most process plants and must be created.

Power module life—Must be taken into consideration as very fast update rates can shorten life, although solutions are now available in terms of energy harvesting devices.

Update time—Fast enough for all monitoring applications, but not fast enough for some real-time control applications.

Training—Training is required for installation and use, but isn’t as extensive as with PROFIBUS PA or FF.

Terminology—New terms and definitions for the wireless world need to be learned, such as active advertising, stability, data reliability, etc.

Troubleshooting—Can be difficult initially until personnel become familiar with wireless networks operation and terminology.

Data—Wireless devices have a plethora of available variable and diagnostic data. For example, Emerson’s Rosemount 3051S Pressure Transmitter has 251 variables, including measured variables, diagnostics, calculated variables, network variables and network diagnostics. A maintenance person needs to be able to sort through this long list and determine what is important and what isn’t.

Pick Your Protocol

As the Table shows, each protocol has its advantages and design considerations, but all share common requirements. Training for engineers implementing the project is essential as is the use of protocol planning tools, engineering guides, segment design recommendations, gateway loading and other design considerations.

Maintenance challenges can be effectively met by training of instrument technicians, purchasing the required tools, and planning for job descriptions that cross between instruments and host systems.

With these requirements satisfied, a blend of wired HART and WirelessHART is often best for projects where the objective is to simplify and speed calibration, instrument maintenance and troubleshooting at the lowest cost.

The HART standard has been in use for decades so many are familiar with installation, start-up, and maintenance of wired HART devices. These instruments can measure multiple process variables or infer variables, but getting these secondary variables out of the instrument can sometimes be a challenge.

PROFIBUS PA is often used in facilities with existing PROFIBUS DP installations, or on projects where the host system offers superior support for PROFIBUS protocols as compared to FF.

FF is the only wired fieldbus network designed from the ground up for field instrument and valve monitoring and control.

WirelessHART makes sense for process plants adding new points of measurement as it requires no signal wiring to instruments, and often no power wiring. Instruments can be added very quickly once the wireless infrastructure is in place, and wiring infrastructure and its required maintenance is much less than with other fieldbus types. In many cases, WirelessHART data transmission will be more reliable than wired systems because there are no wires which can corrode or be inadvertently damaged.

Wired has traditionally been the standard used for control, but wireless is maturing and being relied on more and more for real-time control, depending on application requirements. Going forward, new installations will use a seamless combination of both wired and wireless technologies.

Table: Comparing Fieldbus Types

Fieldbus Attributes

Wired HART

PROFIBUSPA

FOUNDATIONFieldbus

WirelessHART

Purchase Cost

Low

Medium

Medium

High

Installation Cost

Low

High

High

Medium for new installations, very low if wireless infrastructure is in place

Maintenance Cost

Medium

High

High

Low

Reliability

Low

Medium

High

High

Real-Time Control?

No

No

Yes

Yes, except for very high-speed applications

Variety of Available Instruments

High

Low

High

Medium

Retrofit Difficulty

Low

High

High

Low

Access to variables and diagnostic information

Low

High

High

High

About the Author

Dale Perry is the Wireless Service and Training Manager at Emerson Automation Solutions. He has over 32 years of service, training and product management experience in the process industries. Dale is a technical developer and instructor, and he leads worldwide seminars covering Emerson products and industry technologies.